This invention relates to a method of and apparatus for treating BOD-containing water by oxygenation. The BOD-containing water may for example be municipal sewage, chemical waste from petrochemical or paper plants, or fermentation liquor.
With few exceptions, biochemical oxidation methods have employed air as the oxygen source. The large quantity of air required to supply the necessary oxygen is largely due to the 4/1 dilution with nitrogen, and typically only 5-10% of the potential oxygen mass transfer efficiency of the method is attained. However, the air is "free" and the large amount of energy supplied to the air is normally sufficient to mix and suspend the bacterial solids (active biomass) in the liquid.
The direct use of oxygen instead of air in treatment of municipal and chemical wastes has been considered for many years because of its potential advantages in reducing the quantity of gas required. Moreover, it has been speculated that the rate and completeness of such biochemical reactions are suppressed by low dissolved oxygen (DO) levels in the liquor. Because of the additional cost of oxygen, it must be used sparingly and effectively. This necessitates a small volumetric ratio of gas-to-liquor as compared to air aeration. Also, the partial pressure of oxygen in the aerating gas must be sustained at high level to achieve economies in the cost and operation of aeration equipment while still obtaining high overall levels of oxygen utilization. The prior art has not discovered a method which maintains high oxygen partial pressure in aeration while simultaneously utilizing a high percentage of the oxygen contained in the valuable gas. Convention air aeration techniques do not satisfy these requirements.
Other conventional gas-liquid contacting techniques such as packed or plate-type columns, sparged columns, or agitated gas-liquid columns which are commonly employed in chemical processing are not well suited for this particular purpose. Although these systems can be designed to achieve a high percentage oxygen absorption, they are not readily adapted to the handling of mixed liquid-solid suspensions such as encountered in activated sludge processes for waste water treatment. Neither are the conventional systems suited for contacting large volumes of liquor and small volumes of gas with high rates of dissolution and with low energy consumption.
The achievement of both high oxygen utilization and high oxygen partial pressure in biochemical oxidation processes is further complicated by the evolution of diluent gases from the mixed liquor undergoing aeration. Usually the BOD-containing feed water to the process is nitrogen-saturated with respect to air. While mass transfer of nitrogen is not a consideration when air aeration is employed, it becomes a very significant factor when the nitrogen content of the aeration gas is reduced and the volume of aeration gas becomes small. This is because the dissolved nitrogen will be stripped from the liquor into the gas and will reduce the oxygen partial pressure of the gas. Other gases evolved from the liquor which are inert to the biochemical reaction will have a similar effect, e.g., argon and moisture. Carbon dioxide, which is a product of the oxidation, will also evolve in substantial quantity and further suppress the oxygen partial pressure.
If the oxygen-enriched aeration gas is utilized effectively, then its volume relative to air will be very low, e.g., 1/90. While this offers opportunities for cost savings in gas compression, it aggravates the problems of liquid mixing and of oxygen dilution with impurities. The total energy input to the small quantity of gas for purposes of oxygen solution may now be far less than that required for suspending and mixing the solids in the liquid. The inert gases evolved from the liquor will also impair the oxygen partial pressure to a greater extent as the quantity of aeration gas is reduced.
It is an object of this invention to provide an improved system for treating BOD-containing water with oxygen gas for biochemical oxidation.
Another object is to provide a system characterized by high rate of oxygen transfer to the BOD-containing water per unit of energy input, which represents a substantially higher energy transfer efficiency as compared to conventional atmospheric air aeration techniques.
Still another object is to provide a system for oxygenation of BOD-containing water characterized by high oxygen partial pressure and high oxygen utilization efficiency.
Other objects and advantages of this invention will be apparent from the ensuing disclosure and appended claims.